1. Field of the Invention
This invention relates to transducers, and in particular to a capacitance transducer for providing distance or position information about a target relative to the transducer.
1. Discussion of Prior Art
Typically, a capacitance position transducer is used to provide information about the position (or proximity) of an object (or target) relative to the transducer. A conventional transducer includes a single electrode plate for forming a capacitive coupling with the target. The amount of capacitive coupling is inversely proportional to the distance between the electrode plate and the target. The capacitance, and hence the distance, can be detected by means of an oscillator whose frequency is dependent on the capacitance value.
A typical oscillator circuit 1 is shown in FIG. 1 of the accompanying drawings. The electrode plate 3 is coupled to the inverting input of an operational amplifier 5. Negative feedback is provided via a first resistor 7. The positive input of the operational amplifier 5 is fed by a potential divider formed by two resistors 9,11 coupled in series across the output of the amplifier 5.
The transducer operates because the target (not shown) is substantially earthed, or is coupled relative to earth. The transducer effectively responds to the relatively small capacitance between the electrode plate 3 and the target, in series with the capacitance of the target to earth.
With reference to FIG. 2, a problem can arise if more than one such transducer 1 is used in conjunction with a single target 13, for example, to measure the distances between the transducers 1 and the target 13 around the target 13. The problem is caused by cross talk between the individual transducer capacitor signals. Further, if C.sub.T represents a typical value of each transducer capacitance, and C.sub.E represents the target to earth capacitance, then for less than 1% cross talk the value of C.sub.E must be at least one hundred times the value of C.sub.T ; even this factor may not be sufficient in practice.